1. Field of the Invention
The present invention relates to methods for manufacturing high-frequency circuit boards made of porous materials and particularly relates to methods for manufacturing porous dielectric substrates including patterned electrodes. Such porous dielectric substrates are useful in manufacturing microstrip circuit boards for high-frequency devices such as antennas and converters for microwaves and millimeter waves.
2. Description of the Related Art
An electronic circuit board including a dielectric substrate having patterned electrodes arranged thereon is usually manufactured by the following procedure: a conductive layer is provided on a dielectric substrate made of a glass fiber sheet impregnated with a fluorine resin and the patterned electrodes (patterned conductors) are then formed by pattering the conductive layer using a wiring pattern. The conductive layer is usually formed in such a manner that a piece of rolled or electrolytic copper foil is joined to the dielectric substrate by thermal fusion.
However, such a piece of copper foil cannot be joined to a ceramic substrate by thermal fusion. Therefore, a metal material is provided on the ceramic substrate by a paste-firing process (a printing process), a sputtering process, or a PVD process such as a vapor deposition process, whereby patterned electrodes are formed on the ceramic substrate. Since the use of these processes leads to an increase in manufacturing cost and a reduction in productivity, the following process has been recently used: a conductive layer is formed on a ceramic dielectric substrate by a direct wet plating process. In this process, the ceramic dielectric substrate is surface-treated depending on the composition of the ceramic dielectric substrate and the conductive layer is formed directly on the resulting ceramic dielectric substrate by electroless plating. The conductive layer on the ceramic dielectric substrate is patterned by a known process, whereby the patterned electrodes are formed.
In recent years, in order to reduce the dielectric loss of feeder lines, a material with low dielectric loss has been used to manufacture circuit boards for high-frequency circuit devices, such as antennas and converters for microwaves and millimeter waves, including microstrip circuits. Examples of such a material include a porous dielectric material, such as porous silica, having high porosity. However, there is a problem in that cracks are formed in the dielectric material during the fusion of the copper foil piece. Therefore, on an industrial scale, a conductive layer is formed on the dielectric material by the direct wet plating process.
On the other hand, in order to prepare the porous dielectric material at low temperature without using a firing process, the following procedure is used: a skeletal component such as alkoxysilane and a pore-forming component such as a surfactant are mixed with water, whereby an aerogel solution containing the skeletal component hydrolyzed and the pore-forming component is prepared; the solution is applied onto a support plate and the resulting support plate is dried, whereby a primary formed body is formed on the support plate; and the pore-forming component is then extracted from the primary formed body using a supercritical fluid. Alternatively, the following procedure is used: a primary formed body is formed on a base member using a xerogel solution and a solvent contained in this solution is removed from the primary formed body by drying.
However, if the conductive layer is formed on the porous dielectric material by the direct wet plating process, problems below arise and a satisfactory high-frequency circuit board cannot therefore be prepared.
(1) Change in dielectric properties due to high water absorption
Since the porous dielectric material has high porosity and therefore has high water absorption, a plating solution penetrates the material if a wet process is used; hence, the dielectric constant and dielectric loss of the material are varied, that is, the material cannot have properties suitable for high-frequency circuits even if the material is dried.
(2) Diffusion of conductive material for forming patterned electrodes
There is a problem in that a conductive material is diffused into the dielectric material during the formation of the conductive layer by a plating or sputtering process and this causes a deterioration in the flatness of the interface between the dielectric material and the conductive layer, resulting in a deterioration in the accuracy of an electrode pattern.